David Notkin  Autumn 2009  CSE303 Lecture 8

C David Notkin Autumn 2009  CSE303 Lecture 8

Lecture summary • History and characteristics of C • Major C language features • differences between C and Java • basic console input and output (printf and scanf) • Our learning objectives in C • procedural programming • deeper understanding of program compilation and execution • learn details of memory management • debugging skills • software development strategies

History • Created in 1972 by Dennis Ritchie of Bell Labsto accompany the Unix operating system • latest version standard: "C99" (1999) • Designed for creating system software(programs close to the OS that talk directly to hardware) • Also designed to be hardware-independent (portable) • C is also used to develop high-level applications • Currently one of the top two most widely used language worldwide • Based on ALGOL; has influenced the designs of many languages • C++, Java, C#, Perl, Eiffel, Objective-C, Modula, Pascal, ...

Characteristics of C • fairly similar basic syntax and semantics to Java • if/else, for, while,int, double, {} [] () ; +- */% ++ • Much smaller provided standard library than Java • More low-level (more work for programmer, less for compiler) • Procedural (not object-oriented) • C does not have objects as we know them • verb(noun); rather than noun.verb(); • More unsafe (an incorrect program can cause more damage): C programs have more direct access to the system / hardware

First C program #include <stdio.h> int main(void) { printf("Hello, world!\n"); return 0; } • Kernighan and Ritchie started the convention that the first program you show in a new language should be one that prints "Hello, world!"

Dissecting Hello World like import in Java;links the program tothe standard I/O library(includes printf function) #include <stdio.h> int main(void) { printf("Hello, world!\n"); return 0; } the main function header;you don't need to say public staticbecause these are the default in Cmain returns an int error code to the OS(0 on success, > 0 on failure) like println in Java (actually more like System.out.printf);prints output to console

Second C program /* Computes greatest common divisor (GCD) with Euclid's algorithm. */ #include <stdio.h> int main(intargc, char** argv) { int a, b, temp, r; printf("Please enter two positive integers: "); scanf("%d %d", &a, &b); if (b > a) { temp = a; a = b; b = temp; } while ((r = a % b) != 0) { a = b; b = r; } printf("The GCD is %d.\n", b); return 0; }

Compiling/running • To compile a program • gcc -o target source.c • target is the name of the executable program to build • The compiler builds an actual executable file, not a .class like Java • example: gcc -o hi hello.c • To run your program, just execute that file • example: ./hi

gcc options (partial) -Waddress -Warray-bounds (only with ‘-O2’) -Wc++0x-compat -Wchar-subscripts -Wimplicit-int -Wimplicit-function-declaration -Wcomment -Wformat -Wmain (only for C/ObjC and unless ‘-ffreestanding’) -Wmissing-braces -Wnonnull -Wparentheses -Wpointer-sign -Wreorder -Wreturn-type -Wsequence-point -Wsign-compare (only in C++) -Wstrict-aliasing -Wstrict-overflow=1 -Wswitch -Wtrigraphs -Wuninitialized -Wunknown-pragmas -Wunused-function -Wunused-label -Wunused-value -Wunused-variable -Wvolatile-register-var • Most common usage for this course: • gcc -g -Wall -o target source.c • the warnings from -Wall will protect usfrom unwise idioms Guess: how many pages does the gcc manual have on gcc options?

printf • printf("format string", parameters); • A format string contains placeholders to insert parameters into it: • %d or %i an integer • %lf a double ('long floating-point') • %s a string • %p a pointer (seen later) intx = 3; inty = 2; printf("(%d, %d)\n", x, y); // (3, 2)

printf continued • A placeholder can specify the parameter's width or precision: • %8d an integer, 8 characters wide, right-aligned • %-8d an integer, 8 characters wide, left-aligned • %.4f a real number, 4 digits after decimal • %6.2f a real number, 6 total characters wide, 2 after decimal • Examples: int age = 45; double gpa = 1.2345678; printf("%8d %7.3f\n", age, gpa); printf("%8.2f %.1f %10.5f", gpa, gpa, gpa);

Very much the same as Java • General syntax for statements, control structures, function calls • Types int, double, char, long • type-casting syntax • Expressions, operators, precedence + - * / % ++ -- = += -= *= /= %= < <= == != > >= && || ! • Scope (within set of { } braces) • Comments: /* ... */, // ( // not officially legal until C99)

Mostly the same as Java • Variables • can be used without being initialized (!) • must be declared at the start of a function or block (changed in C99) • for loops • variable cannot be declared in the loop header • if/else statements, while and do/while loops • there is no boolean type (changed in C99) • any type of value can be used as a test • 0 means false, every other number means true • Parameters / returns • C has certain features for values vs. references ("pointers")

Very different from Java • Strings • very clunky to use in C; arrays of characters • are not objects; do not contain methods (external string functions) • I/O to/from console and files • no Scanner; must use input functions such as scanf • console I/O different than file I/O • Errors and exceptions • C has no try/catch and does not represent errors as objects • errors are usually returned as integer error codes from functions • crashes are mostly called "segmentation faults" and are not of much direct utility in figuring out what is wrong

Also very different • Arrays • are just bare contiguous blocks of memory • have no methods and do not know their own length (!) • Objects • C doesn't have them • closest similar feature: struct (a set of fields; no methods) • Memory management • most memory that you consume, you must explicitly free afterward • API and provided libraries • C doesn't have very many, compared to Java • you must write many things yourself (even data structures)

scanf • scanf("format string", variables); • uses same syntax for formatted strings, placeholders as printf • Must precede each variable with an & (address-of operator) int x; int y; printf("Type your x and y values: "); scanf("%d %d", &x, &y);

scanf continued • scanf returns the number of values successfully read: can be examined to see whether the reading was successful • if # of variables listed doesn't match # of format placeholders • too many variables: later ones ignored • too few variables: program crashes! • Can match a specific input pattern int x; int y; printf("What is your (x, y) point?\n"); scanf("My point is (%d, %d)", &x, &y);

Practice exercise • Write a C program that makes change: • prompts the user for an amount of money • reports the number of pennies, nickels, dimes, quarters, and dollars • Example Amount of money? 17.93 Pennies : 2 Nickels : 1 Dimes : 1 Quarters: 3 Dollars : 17

Social Implications/Ethics Friday • Forum On Risks To The Public In Computers And Related Systems: http://catless.ncl.ac.uk/Risks • What is an engineering failure? • What are some of your “favorite” failures related to computers and software? • What do we learn from them?

Questions?

David Notkin  Autumn 2009  CSE303 Lecture 8